The present invention relates to a method for compensating for a mechanical oscillation, in particular for a rotational oscillation, which has a frequency spectrum which can be approximately represented by a number of discrete frequency components, at a machine shaft, in particular in a printing unit or in a printing machine, via at least one actuator acting directly or indirectly upon the machine shaft.
Moreover, the present invention relates to a printing unit having at least one machine shaft driven by an actuator and a measuring device for picking up the mechanical oscillation of the machine shaft.
A mechanical oscillation is to be understood both as periodic changes of one or more coordinates of the machine shaft, for example, rotational oscillations, vibrational oscillations or the like, and superpositions of a plurality of periodical changes. The term machine shaft chosen hereinafter denotes a mathematical axis of rotation and, in this context, can be, in particular, any shaft, axle, machine axis, roll, cylinder, or the like, of a machine. In this connection, the term machine shaft will include hereinafter also a virtual axis of rotation, that is a signal calculated from coordinate values of one or a plurality of axes of rotation, in particular the difference between coordinates of two real machine shafts.
In the case of machines having speed or positional control of at least one machine shaft, it is possible that changing torques act upon the controlled shaft because of cam discs, imbalances, or other constructional conditions. Depending on the transmission ratio between the shaft of the original torque and the controlled shaft, oscillations are superposed on the latter whose frequency is proportional to its rotational speed, i.e., has a fixed machine order. Moreover, excitations having a fixed frequency which is independent of the speed of the controlled shaft can be superposed upon the rotary motion. In both situations, the frequency spectrum of the mechanical oscillation of the position, speed and acceleration of the controlled shaft contains disturbing discrete frequencies which will be referred to as disturbances hereinafter, independently of their origin. If the intention is for the machine shaft to rotate at a constant speed, then disturbances give rise to deviations from the setpoint speed or from the rotational setpoint angle which cannot be completely corrected but only reduced to a sufficient degree with increased outlay in connection with the driving mechanism.
Especially in the case of printing machines, whether they are sheet-fed printing presses or web-fed printing presses, the accuracy with which the speed of the controlled shaft is maintained or with which a rotational setpoint position is followed, has a decisive influence on the quality of the product. Disturbances having an integral frequency ratio to the rotational frequency of the controlled shaft of a paper guiding cylinder are generally negligible since they are equal for all print images or print sheets. However, all other frequencies are disturbing because they can give rise to the so-called “ghosting” when, in other words, successive sheets are printed at an offset relative to one another.
If a sheet-fed printing press is composed of a plurality of separately driven, mechanically decoupled parts such as printing units, printing unit groups, or the like, then variations in the angular difference between two successive paper guiding cylinders, that is two controlled shafts, at the sheet transfer between the parts of the printing machine become directly noticeable as variations in the circumferential register. In this context, it is desired for the transfer angle to be identical from sheet to sheet; however, the transfer angle is impaired by oscillations of non-integral order.
Diverse devices and methods for damping mechanical oscillations, in particular rotational oscillations, in printing machines are already known.
European Patent EP 0 592 850 B1 describes a device and a method for damping mechanical oscillations of printing machines. This device features at least one actuating member and one vibration sensor; these can be arranged in a control loop as well. In this context, the damping of printing quality impairing mechanical oscillations in the stock-guiding system of a printing machine is directed to the compensation for asynchronous oscillations, that is only for oscillations which occur non-periodically with the revolutions of the rotating parts.
German Patent Application DE 44 12 945 A1 discloses a device and a method for damping mechanical oscillations of printing machines which also allows integral oscillation orders to be compensated for. The data for driving the actuating members is determined either by calculation or by measurement in a test run of the printing machine.
German Patent Application DE 199 14 627 A1 relates to a method and a device for compensating for a rotational oscillation in a printing machine, the method being designed in such a manner that at least one intrinsic oscillation shape of the printing machine is determined and that a specific counter-torque for compensating for the torques that excite oscillations in the intrinsic shape is applied to at least one location at which this intrinsic shape does not have the amplitude 0.
Further related art is constituted by German Patent Application 197 49 134 A1, wherein an active oscillation damping device and a method for identifying the transfer function in an active oscillation damping device are disclosed. A control device reads in a residual oscillation signal from a residual oscillation detector of the active oscillation damping device synchronously with a predetermined input sampling timer. Subsequent to reading the residual oscillation signal as a time series for each frequency, an FFT (Fast Fourier Transformation) is carried out for each time series to obtain a frequency component of the original sinusoidal wave. A calculation of the inverse FFT is then carried out for the result of the composition of each obtained frequency component in order to derive a pulse response as a transfer function.
Furthermore, European Patent EP 0 425 352 B1 describes a device for actively damping oscillations whose energies are concentrated in frequencies including a fundamental and the harmonic thereof. The device, which is used for damping oscillations of a mechanical part, includes vibration pickups, which pick up characteristic electrical signals, in amplitude and phase, of the vibration at a point of the mechanical part, at least one actuator which is capable of exerting a force on the mechanical part against the vibration, and an arithmetic unit which is connected to the vibration pick-up and to the actuator. The output signals of each vibration pick-up are subjected to a synchronous detection with the aid of reference signals which correspond to the different frequencies which include an energy concentration. To this end, each output signal, after being sufficiently amplified, is subjected to a sampling and to an analog-to-digital conversion for each frequency generated by a synthesizer. The random samples experience a synchronous demodulation including a multiplication and a passage through a low-pass filter and for each retained frequency. The reference signals are obtained with unaltered phase utilizing a linear relation with the fundamental frequency. For each frequency at which energy is concentrated in the mechanical oscillation, a recursive adaptation algorithm is carried out by the arithmetic unit in such a manner that each actuator receives a signal of its own which includes the sum of the contributions of the different frequencies.
German Patent Application 196 14 300 A1 discloses a method for the self-regulating compensation for the effects of the irregular concentric running of a roller or a reel for collar eccentricity compensation in a reel system or the winding up or unwinding of sheet-like material. In this context, the roller rotates at a changed speed due to the varying collar radius or roller radius. The actual draw value is approximated by at least one rotary-harmonic sine function of exclusively integral order whose argument is the rotational angle of the roller, the sinusoidal approximation being carried out according to the orthogonal correlation or in accordance with the harmonic analysis according to Fourier, and estimates being calculated for the amplitude and phase of the sinusoidal signal caused by the irregular concentric running. An additional torque which is calculated from the estimates is added to the setpoint torque value for the roller.
The determination of suitable compensation data, in particular for printing machines, constitutes a problem of compensation methods and compensation devices heretofore, which impedes the broad application thereof. If a storage device is used for the compensation data, then, in common compensation approaches heretofore, a calculation or a test run for measuring suitable compensation data are required in advance. Both methods turn out to be difficult, time and cost intensive. Typical compensation approaches in printing machines consider oscillations including a plurality of frequencies as a whole. Because of this, it is possible only with difficulty to adapt the compensation in a differentiated manner as a function of the machine dynamics, control, disturbance, or the like. In particular, an adaptation to oscillation shapes which vary strongly over time or to a changed machine dynamics can be implemented only with difficulty.
When calculating the oscillations, there is a risk of systematic errors if simplifying or even false assumptions are made. An approach of that kind implies that only oscillations which are accessible to calculation can be compensated for. Moreover, conventional test runs for determining the oscillation involve the risk of measuring errors due to different, further disturbances. When utilizing the determination of the intrinsic shapes of a machine, a machine-specific design is required.